We have now established many of the essential features of sun-mediated cutaneous synthesis of vitamin D3. During exposure to sunlight epidermal 7-dehydrocholesterol (7-DHC) is converted to previtamin D3 (preD3). Once formed previtamin D3 can either thermally isomerize to vitamin D3 or photoisomerize to lumisterol and tachysterol. Because the vitamin-D-binding protein has selective affinity for vitamin D3 it is thought that this protein plays a role in the translocation of vitamin D3 into the circulation. We have developed the methodology to evaluate in human subjects the effect of exposure to UV radiation on circulating levels of vitamin D3 and its metabolites. During the past 3 years we have developed new technology including (1) high resolution high performance liquid chromatography and gas-liquid chromatography methods to separate 7-DHC from other sterols and its photoproducts, (2) tissue culturing of 1,25-(OH)2-D receptor + and - keratinocytes and fibroblast, (3) synthesis of radiolabeled photoproducts of preD3 and vitamin D3 and a light source that mimics natural sunlight. Using these techniques we plan to expand our productive research program and investigate the (1) photoconversion of preD3 and vitamin D3 in human skin to toxisterols and suprasterols and their physiologic consequences, (2) translocation and metabolism of lumisterol and tachysterol in human skin, (3) role of cutaneous 7-DHC-esters in the production of vitamin D3, (4) factors such as UV radiation 7-DHC photoproducts and vitamin D metabolites that could regulate cutaneous 7-DHC production, (5) changes in circulating concentrations of vitamin D3 and its metabolites after exposure to UV radiation in health and disease and (6) therapeutic effectiveness of using the skin as the site of synthesis absorption of vitamin D metabolites.